Process for the production of expandable particles of styrene polymers

ABSTRACT

Process for the production of expandable particles of styrene polymers, having improved processability characteristics, which comprises: i) producing expandable particles of styrene polymers containing from 2 to 20% of an expanding agent incorporated; ii) covering these particles with an antistatic agent in a quantity of less than 1% by weight; iii) vigorously frictioning the particles in a mixing apparatus in order to heat their surface, in a short time, by simple mechanical friction; iv) discharging the particles thus treated from the mixing apparatus.

This application is a Continuation of application Ser. No. 09/015,779filed on Jan. 29, 1998, U.S. Pat. No. 5,935,645.

The present invention relates to a process for the production ofexpandable particles of styrene polymers having improved processabilitycharacteristics.

More specifically, the present invention refers to a process for theproduction of expandable particles or beads of styrene polymersparticularly suitable for the moulding of end-products and blocks havingan excellent surface appearance, with low shrinkage, a high sinteringdegree and with a relatively short residence time in the mould.

The production of expandable particles of styrene polymers, inparticular polystyrene, is well known in the art. It schematicallycomprises the following phases:

inorporation in the polymer, in the polymerization phase, or after, ofan expanding agent which boils at a temperature lower than the softeningpoint of the polymer;

washing of the polymer and centrifugation;

drying with air;

screening with the separation of homogeneous fractions (with thepossible help of an antistatic agent);

deposition of the coating;

packaging.

The transformation process of expandable styrene polymers is also wellknown in the art. It essentially consists of the following operativesteps:

pre-expansion with vapor at 100-105° C. The expansion is obtained byheating to a temperature higher than the boiling point of the expandingagent and softening point of the styrene polymer;

aging of the pre-expanded particles for 24 hours at room temperature;

final moulding with vapor. For the production of low density expandedblocks and end-products, the pre-expanded particles are arranged in thecavity of a mould which defines the shape of the desired end-article andare subsequently heated to a temperature higher than the boiling pointof the expanding agent and softening point of the polymetric material.During this heating, the particles expand and sinter, owing to thelimited space available, to form a body having the shape and dimensionsof the cavity of the mould used. After the moulding, the shaped body isleft to cool inside the mould for a sufficiently long period of time, toavoid deformation once extracted from the mould. As the expanded plasticmaterial is an excellent thermal insulator, relatively long residencetimes in the mould are required to cool the shaped body;

evaluation of the moulded end-products in terms of appearance, sinteringdegree, etc.

It is known that to improve the processability of the expandedpolystyrene particles, it is convenient to subject these, before thedeposition of the coating, to thermal treatment, in the presence of atleast one antistatic agent, which induces a considerable loss ofexpanding agent but at the same time produces an improvement in theproperties such as:

low residence time in the mould;

low shrinkage of the moulded end-products after extraction from themould;

high sintering degree;

reduced aging times;

high absorbing capacity of the pigments or other surface additives.

This treatment presumably causes modifications to the surface of thepolymer which, in turn, determine the above improvements.

For instance, published European patent application 518.140 describes aprocess for the thermal treatment of expandable particles of styrenepolymers, containing from 2 to 20% by weight of an expanding agentincorporated, which consists in:

covering these particles with an antistatic agent, for example withethylene oxide-propylene oxide block copolymers having a content ofethylene oxide ranging from 10 to 50% by weight and having an averagemolecular weight of between 1,000 and 5,000, in a quantity not higherthan 1% by weight;

removing a part of the expanding agent incorporated in the particles, byheating these to a temperature lower than the glass transitiontemperature of the styrene polymer;

rapidly cooling the particles to stop the evaporation of the expandingagent;

depositing the coating according to the usual procedures.

The expanded particles thus treated have very interestingcharacteristics, such as:

a) low residence time in the mould, generally equal to or less than 50%of the time necessary for cooling the blocks or end-products obtainedfrom non-treated particles or particles treated with traditionalsystems;

b) shrinkage of the blocks or end-products, after extraction from themould, of less than 1%;

c) a high sintering degree of the expanded particles;

d) short aging times after the pre-expansion phase;

e) high absorption capacity of pigments or other surface additives.

However, the partial removal of the expanding agent by heating to atemperature lower than the glass transition temperature of the material,for example between 35 and 50° C., also has its disadvantages. In fact,the loss of expanding agent which occurs with this operation is between5 and 50% by weight of the total agent incorporated in the particle.

This loss reduces the possibility of preparing expanded products with areduced density and also requires the necessity of recovering theexpanding agent itself, generally with an aliphatic hydrocarbon with alow boiling point such as pentane or hexane, which must be recoveredboth for reasons of cost and environmental hygiene.

In addition, the same removal operation of the expanding agent requiresvery high treatment times and this greatly reduces the productivity.

The Applicant has now found a process for improving the processabilityof expanded particles of styrene polymers which overcomes the abovedrawbacks without reducing the properties (a)-(e) which can be obtainedwith the systems of the known art. This result has been made possible inthat a new and inventive method has been found which enables the thermaltreatment of the particle to be limited only to the surface of theparticles themselves.

The present invention therefore relates to a process for the productionof expandable particles of styrene polymers, having improvedprocessability characteristics which comprises:

i) producing expandable particles of styrene polymers containing from 2to 20% of an expanding agent incorporated;

ii) covering these particles with an antistatic agent in a quantity ofless than 1% by weight (immediately before or after a screening phase);

iii) vigorously frictioning the particles in a mixing apparatus in orderto heat their surface, in short times, by simple mechanical friction;

iv) discharging the particles thus treated from the mixing apparatus.

After following the operating steps (i)-(iv), the expandable productsare ready to be treated with the coating according to the conventionalprocedures.

Owing to the specific treatment object of the present invention, i.e. asimple mechanical operation on the surface of the particles, and due tothe poor thermal conductivity of styrene polymers, in particularpolystyrene, the heating is limited to a small surface layer of theparticles and therefore the increase in temperature is almost immediate.

In experimental practice it is not possible to measure the temperatureincrease which takes place on the surface of the beads, even if it canbe estimated as being much higher than 100° C. In reality, what ismeasured with the thermometer is the temperature of the bulk immediatelyafter the end of the treatment.

In particular, the benefits in the properties in transformation aresignificant results when the temperature of the bulk (mass of particles)is between 20 and 55° C.

According to the process of the present invention, owing to theparticular treatment and poor conductivity of the polymer, the increasein temperature basically causes a partial removal of expanding agentwhich is limited to the surface layer in question of the particles. Infact, styrene polymers, and in particular polystyrene, have a lowthermal conductivity and therefore, also thanks to the short frictiontreatment times, the body of the particle does not undergo any heatingeffect and consequently no loss of expanding agent.

The particles containing the expanding agent incorporated can beproduced by carrying out the polymerization, preferably in aqueoussuspension, of the styrene monomer, alone or mixed with one or moreethylenically unsaturated monomers copolymerizable with this, in thepresence of the expanding agent. Other production methods are productionin resuspension and in mass.

The polymerization can be carried out in the presence of at least oneperoxidic initiator or thermally, according to the known andconventional processes.

Suspending agents available on the market can be used in the process ofthe present invention. The examples include products soluble in watersuch as polyvinylalcohol, methylcellulose, polyvinylpyrrolidone, etc.and products not very soluble such as magnesium pyrophosphate or calciumtriphosphate.

Alternatively, the expanding agent can be incorporated into theperformed styrene polymer by exposing the particles to the vapors of theexpanding agent, or by introducing it into the particles already formedsuspended in water or in extrusion. The various methods for theproduction of expandable particles are well known to experts in thefield and are described in literature, for example in British patents695.826; 715.100; 886.811; 908.089; 1.048.243; or in U.S. Pat. No.2,983,692, whose contents form an integrant part of the presentinvention.

The term "styrene polymers" as used in the present invention and claims,comprises both the homopolymer of styrene and copolymers of styrene withother vinyl and/or vinylidene comonomers, containing at least 50% byweight of chemically combined styrene. Examples of these comonomers areα-methylstyrene; vinyltoluene; p-methylstyrene; p-ethylstyrene; styreneshalogenated in the nucleus, such as 2,4-dichlorostyrene; acrylonitrile;methacrylonitrile; esters of α, β-unsaturated carboxylic acids withalcohols containing from 1 to 8 carbon atoms, such as esters of acrylicand/or methacrylic acid; N-vinyl compounds, such as vinylcarbazole;anhydrides such as maleic, phthalic anhydride; etc.

The term copolymers of styrene also includes copolymers containing, aswell as styrene and possibly the vinyl and/or vinylidene comonomersmentioned above, also small quantities of monomers containing two doublebonds of the vinyl type, such as divinylbenzene.

The expandable particles of styrene polymers contain as expanding agentsconventional organic compounds which are easily volatile, gaseous orliquid at room temperature, which do not dissolve the polymer but expandit and whose boiling point is lower than the softening point of thepolymer.

Examples of particularly suitable expanding agents are aliphatichydrocarbons, alone or mixed with each other, containing from 2 to 6carbon atoms such as propane, butane, n-pentane, i-pentane, hexane,cyclohexane, etc. petroleum ether; halogenated derivatives of aliphatichydrocarbons containing from 1 to 3 carbon atoms, such as the variouschloro and fluoro derivatives of methane, ethane or ethylene, forexample dichlorodifluoromethane; 1,2,2-trifluoroethane;1,1,2-trifluoroethane; carbon dioxide; etc.

The expanding agent is generally used in a quantity ranging from 2 to20% and, preferably, from 4 to 10% by weight with respect to thepolymer.

As well as the expanding agents, the polymers of styrene can containother additives, such as for example flame-extinguishing agents, organicand inorganic loadings, dyes, pigments, antipacking agents,plasticizers, etc.

The polymerization of styrene, preferably in aqueous suspension, theaddition of the expanding agent, preferably during the polymerizationand the transformation of the particles into shaped bodies, by mouldingin closed forms, are carried out according to the techniques well knownto experts in the field, which are widely described in literature, suchas for example in "Rigid Plastic Foams", by T. N. Ferrigno, ReinholdPublishing Corp. New York, U.S.A. (l963).

The antistatic agents used in the process of the present invention arethose generally proposed for improving the flowability; finish andprocessability surface properties of the particles of styrene polymers.

These antistatic agents are well known and described in literature, forexample in Kirk-Othmer, ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Vol. 3,1992, page 540. Examples of antistatic agents which can be used in theprocess of the present invention comprise: esters of fatty acids, suchas for example butyl stearate; mono- or poly-hydric alcohols, such asfor example glycerol; amines, such as for example an ethyoxylatedtertiary alkylamine, a dialkanolamine of fatty acids; amides, such asfor example N,N-bis-(2-hydroxyethyl-stearamide); polyoxyethylene orpolyoxyalkylene derivatives, such as for example hexadecylether ofpolyethyleneglycol; ethylene oxide-propylene oxide copolymers; aminicsoaps, such as for example the salt of stearic acid and octadecylamine;alkylsulfated aminicides, such as for example the salt of guanidine andoctadecylsulfate; compounds of quaternary ammonium, such as for exampleoctadecyl-trimethylammonium chloride; alkylphosphates, such as forexample acid bis-dodecylphosphate; aminic salts of alkylphosphonicacids, such as for example the salt of triethanolamine, andoctadecylphosphonic acid; etc.

These antistatic agents can be used alone or mixed with each other inany ratio.

The preferred antistatic agents in the process of the present inventionare: fatty alcohols condensed with ethylene oxide and propylene oxideknown on the market under the trade-name DEHYPON® of Henkel orethoxylated tertiary alkylamines such as the product HOSTASTAT® ofHoechst.

The quantity of antistatic agent is generally not more than 1% by weightwith respect to the polymer, for example from 0.001 to 0.5% andpreferably from 0.01 to 0.1% .

The coating of the polymer particles with the antistatic agent can becarried out using any known mixing method. For example, the expandableparticles can be treated with the appropriate quantity of antistaticagent in a rotating drum or screw mixer.

The coating of the particles with the antistatic agent facilitatessubsequent screening operations, sometimes necessary for obtainingfractions, each with homogeneous dimensions, destined for various kindsof use.

The vigorously frictioning operation consists in inducing strongfriction of the particles against each other and together against thewalls of the device used for this purpose. Any device capable ofvigorously frictioning polymeric particles can be used in the process ofthe present invention. A suitable device may be a screw mixer, withrotating blades, etc., capable of operating both in continuous and inbatch, normally used as a means for mixing additives such as dyes,stabilizers, coatings, etc. with thermoplastic polymers such as PVC,ABS, polyethylene, polypropylene, expandable polystyrene, etc.

In any case the device used must be such as to allow a rapid heating ofthe surface in a time range which varies in relation to the dimensionsof the particles and the type of device itself (device for operating incontinuous or batch).

In the functioning phase, therefore, the polymeric particles chargedundergo a strong friction which causes an increase in the temperature onthe surface of each of these with a consequent release of the expandingagent. As the heating effect, and therefore the evaporating effect ofthe expanding agent, remains solely limited to the surface of the bead,the loss in expanding agent is very much limited but, inspite of this,it guarantees the acquisition of the desired properties.

Some illustrative but non-limiting examples are provided for a betterunderstanding of the present invention and for its embodiment.

EXAMPLE 1 (COMPARATIVE)

20 Kg of particles of polystyrene having a weight average molecularweight of 180,000, an average diameter ranging from 0.2 to 2.7 mm andcontaining an expanding agent incorporated consisting of a mixture ofn-pentane and iso-pentane in a weight ratio 70/30, were obtained by thepolymerization of styrene, in an aqueous suspension, at a temperatureranging from 85 to 120° C. and in the presence of dibenzoyl peroxide andter-butyl perbenzoate as catalysts.

The particles leaving the polymerization reactor were washed with water,centrifuged and dried at 20° C. for 1 hour. The content of expandingagent measured was 6.8% by weight.

300 ppm of an antistatic agent were added to the expandable particles,consisting of a fatty alcohol condensed with ethylene oxide andpropylene oxide having a ratio in moles of 5:4 and a molecular weightranging from 500 to 700, known on the market as DEHYPON LS®. Theaddition was carried out in a screw mixer, at room temperature, for aperiod of about 10 prime minutes.

The particles were then screened, separating the fraction with anaverage diameter ranging from 0.4 to 0.9 mm.

A mixture of coating agents consisting of 0.2% by weight with respect tothe polymer, of glyceryl mono-stearate and 0.05% of zinc stearate wereadded to the particles, which were then pre-expanded with vapor at95°-100° C. until the minimum density of 16 g/l was reached. After agingin air at room temperature for 24 hours, the particles were moulded toproduce containers having dimensions of 40×60×2 cm. The moulding wascarried out with vapor at 0.9 Ate.

The containers thus obtained had a sintering degree, referring to thepercentage of expanded particles which split after breakage of thebottom of the container, of 15%. The sintering percentage was determinedby defining a breakage section of about 10 cm², counting the totalnumber of expanded particles in this section and calculating the numberof those which were broken with respect to the total number.

The cooling time of the containers was determined, intended as theresidence time in the mould necessary for the end-product to maintainthe dimensions of the mould once extracted. The cooling time of thecontainers proved to be 5 prime minutes.

EXAMPLE 2 (Comparative)

The same procedure was adopted as in example 1 up to the screening,separating the fraction with an average diameter ranging from 0.4 to 0.9mm.

The particles were then charged into an oven and heated to 35° C. untilthe content of expanding agent was reduced to 5% by weight. A mixture ofcoating agents was added to the particles, consisting of 0.2% by weight,with respect to the polymer, of glyceryl monostearate and 0.05% of zincstearate, and the particles were then pre-expanded with vapor at 95-100°C. until the minimum density of 22 g/l was reached.

After aging in air at room temperature for 24 hours, the particles weremoulded as described in example 1. The sintering degree proved to be 80%and the cooling time 2'45".

EXAMPLE 3

The same procedure was adopted as in example 1 up to the screening,separating the fraction with an average diameter ranging from 0.4 to 0.9mm. The content of pentane was 6.8% and the temperature 23° C.

The particles were fed to a counter-rotating blade mixer and processeduntil a temperature of 35° C. was reached. The material was thendischarged and left to cool for 20' at room temperature. The content ofpentane was 6.5%.

A mixture of coating agents was added to the particles thus treated,consisting of 0.2% by weight, with respect to the polymer, of glycerylmonostearate and 0.05% of zinc stearate, and the particles were thenpre-expanded with vapor at 95-100° C. until the minimum density wasreached (17 g/l).

After aging in air at room temperature for 24 hours, the particles weremoulded to produce containers having dimensions of 40×60×2 cm. Themoulding was carried out with vapor at 0.9 Ate.

The containers thus obtained had a sintering degree of 85% and a coolingtime of 2'30".

EXAMPLE 4

The same procedure was carried out as in example 3 up to thepre-expansion with vapor at 95-100° C. until the minimum density wasreached (17 g/l).

After aging in air at room temperature for 5 hours, the particles weremoulded to produce containers having dimensions of 40×60×2 cm. Themoulding was carried out with vapor at 0.9 ate.

The containers thus obtained had a sintering degree of 70% and a coolingtime of 3'.

EXAMPLE 5 (Comparative)

The same procedure was adopted as in example 1 up to the screening,separating the fraction with an average diameter ranging from 0.9 to 2.7mm. The content of pentane was 6.9%.

A mixture of coating agents was added to the particles thus treated,consisting of 0.2% by weight, with respect to the polymer, of glycerylmonostearate and 0.05% of zinc stearate, and the particles were thenpre-expanded with vapor at 95-100° C. until the minimum density wasreached (14 g/l).

After aging in air at room temperature for 24 hours, the particles weremoulded to produce blocks having dimensions of 100×100×50 cm. Themoulding was carried out with vapor at 0.65 ate

The blocks thus obtained had a sintering degree of 5% and a cooling timeof 20'.

EXAMPLE 6 (Comparative)

The same procedure was adopted as in example 1 up to the screening,separating the fraction with an average diameter ranging from 0.9 to 2.7mm. The content of pentane was 6.9%.

The particles were then charged into an oven and heated to 35° C. untilthe content of expanding agent was reduced to 5.1% by weight. A mixtureof coating agents was added to the particles, consisting of 0.2% byweight, with respect to the polymer, of glyceryl monostearate and 0.05%of zinc stearate, and the particles were then pre-expanded with vapor at95-100° C. until the minimum density of 20 g/l was reached.

After aging in air at room temperature for 24 hours, the particles weremoulded to produce blocks having dimensions of 100×100×50 cm. Themoulding was carried out with vapor at 0.65 ate.

The blocks thus obtained had a sintering degree of 50% and a coolingtime of 9'.

EXAMPLE 7

The same procedure was adopted as in example 1 up to the screening,separating the fraction with an average diameter ranging from 0.9 to 2.7mm. The content of pentane was 6.9% and the temperature 23° C.

The particles were fed to a counter-rotating blade mixer and processeduntil a temperature of 35° C. was reached. The material was thendischarged and left to cool for 20' at room temperature. The content ofpentane was 6.6%.

A mixture of coating agents was added to the particles thus treated,consisting of 0.2% by weight, with respect to the polymer, of glycerylmonostearate and 0.05% of zinc stearate, and the particles were thenpre-expanded with vapor at 95-100° C. until the minimum density wasreached (15 g/l).

After aging in air at room temperature for 24 hours, the particles weremoulded to produce blocks having dimensions of 100×100×50 cm. Themoulding was carried out with vapor at 0.65 ate.

The blocks thus obtained had a sintering degree of 55% and a coolingtime of 8.30'.

The results of examples 1-7 are summarized in tables I and II.

                  TABLE 1                                                         ______________________________________                                        Initial   Final   Minimum                                                     pentane   pentane density  Sintering                                                                            Cooling                                                                              Aging                                %         %       g/l      %      time   hrs                                  ______________________________________                                        Ex. 1 6.8     6.8     16     15     5'     24                                 comp.                               (0.9 Ate)                                 0.4-0.9                                                                       mm                                                                            Ex. 2 6.8     5.0     22     80     2'45"  24                                 comp.                               (0.9 Ate)                                 0.4-0.9                                                                       mm                                                                            Exam- 6.8     6.5     17     85     2'30"  24                                 ple 3                               (0.9 Ate)                                 0.4-0.9                                                                       mm                                                                            Exam- 6.8     6.5     17     70     3'      5                                 ple 4                               (0.9 Ate)                                 0.4-0.9                                                                       mm                                                                            ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Initial   Final   Minimum                                                     pentane   pentane density  Sintering                                                                            Cooling                                                                              Aging                                %         %       g/l      %      time   hrs                                  ______________________________________                                        Ex. 5 6.9     6.9     14      5     20'    24                                 comp.                               (0.65 Ate)                                0.9-2.7                                                                       mm                                                                            Ex. 6 6.9     5.1     20     50      9'    24                                 comp.                               (0.65 Ate)                                0.9-2.7                                                                       mm                                                                            Exam- 6.9     6.6     15     55     8'30"  24                                 ple 7                               (0.65 Ate)                                0.9-2.7                                                                       mm                                                                            ______________________________________                                    

The Italian priority application No. MI97A 000173 is herein incorporatedby reference.

What is claimed is:
 1. A process for the production of expandableparticles of styrene polymers, having improved processabilitycharacteristics, which comprises:i) producing expandable particles ofstyrene polymers containing from 2 to 20% by weight of an expandingagent; ii) coating these particles with an antistatic agent in aquantity of less than 1% by weight; iii) subsequent to said coating, andin a separate step, vigorously frictioning the particles in a mixingapparatus in order to heat their surface, by simple mechanical frictionalone; iv) discharging the particles thus treated from the mixingapparatus.
 2. The process according to claim 1, wherein the friction issuch as to heat the mass of particles to a temperature ranging from 20to 55° C.
 3. The process according to claim 1, wherein the particles ofstyrene polymers containing the expanding agent are produced by carryingout the polymerization in an aqueous suspension of a styrene monomer,alone or mixed with one or more ethylenically unsaturated monomerscopolymerizable with this, in the presence of the expanding agent. 4.The process according to claim 3, wherein the suspending agents areselected from the group consisting of products soluble in water such aspolyvinylalcohol, methyl-cellulose, polyvinylpyrrolidone and not verysoluble products such as magnesium pyrophosphate or calciumtriphosphate.
 5. The process according to claim 3, wherein the expandingagent is selected from the group consisting of aliphatic hydrocarbons,alone or mixed with each other, containing from 2 to 6 carbon atoms;petroleum ether; halogenated derivatives of aliphatic hydrocarbonscontaining from 1 to 3 carbon atoms; and carbon dioxide.
 6. The processaccording to claim 1, wherein the antistatic agents are selected fromthe group consisting of the esters of fatty acids; mono- or polyhydricalcohols; amines; amides; polyoxyethylene or polyoxyalkylenederivatives; ethylene oxidepropylene oxide copolymers; aminic soaps;alkylsulfated aminicide salts; compounds of quaternary ammonium;alkylphosphates; and aminic salts of alkylphosphonic acids; and mixturesthereof.
 7. The process according to claim 1, wherein the quantity ofantistatic agent is between 0.001 and 0.5% by weight.
 8. The processaccording to claim 1, wherein the vigorous friction operation is carriedout by means of a screw mixer or a rotating blade mixer.
 9. The processaccording to claim 6, wherein the antistatic agents are selected fromthe group consisting of butyl stearate, glycerol, ethoxylated tertiaryalkylamine, fatty acid dialkylanolamine, N,N-bis-(2-hydroxyethylstearamide), hexadecylether of polyethyleneglycol,salt of stearic acid and octadecylamine, salt of guanidine andoctadecylsulfate, octadecyl-trimethylammonium chloride, acidbis-dodecylphosphate, and salt of triethanolamine andoctadecylphosphonic acid, and mixtures thereof.
 10. The processaccording to claim 1, wherein the antistatic agent is selected from thegroup consisting of fatty alcohols condensed with ethylene oxide, fattyalcohols condensed with propylene oxide, and ethoxylated tertiaryalkylamines, and mixtures thereof.